linux-hardening - Re: [PATCH v5] overflow: Introduce overflows_type() and castable_to

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Message-ID: <307b90cb-b80d-6ce3-14ae-4a0b2ee5e447@prevas.dk>
Date:   Wed, 2 Nov 2022 12:52:32 +0100
From:   Rasmus Villemoes <rasmus.villemoes@...vas.dk>
To:     Gwan-gyeong Mun <gwan-gyeong.mun@...el.com>, keescook@...omium.org
Cc:     luc.vanoostenryck@...il.com, nathan@...nel.org,
        ndesaulniers@...gle.com, trix@...hat.com, dlatypov@...gle.com,
        vitor@...saru.org, gustavoars@...nel.org,
        linux-hardening@...r.kernel.org, llvm@...ts.linux.dev,
        jani.nikula@...ux.intel.com, joonas.lahtinen@...ux.intel.com,
        rodrigo.vivi@...el.com, tvrtko.ursulin@...ux.intel.com,
        airlied@...ux.ie, daniel@...ll.ch, linux-kernel@...r.kernel.org,
        intel-gfx@...ts.freedesktop.org, dri-devel@...ts.freedesktop.org,
        linux-sparse@...r.kernel.org, arnd@...nel.org, mchehab@...nel.org,
        mauro.chehab@...ux.intel.com
Subject: Re: [PATCH v5] overflow: Introduce overflows_type() and
 castable_to_type()

On 24/10/2022 22.11, Gwan-gyeong Mun wrote:
> From: Kees Cook <keescook@...omium.org>
> 
> Implement a robust overflows_type() macro to test if a variable or
> constant value would overflow another variable or type. This can be
> used as a constant expression for static_assert() (which requires a
> constant expression[1][2]) when used on constant values. This must be
> constructed manually, since __builtin_add_overflow() does not produce
> a constant expression[3].
> 
> Additionally adds castable_to_type(), similar to __same_type(), but for
> checking if a constant value would overflow if cast to a given type.
> 

> +#define __overflows_type_constexpr(x, T) (			\
> +	is_unsigned_type(typeof(x)) ?				\
> +		(x) > type_max(typeof(T)) ? 1 : 0		\
> +	: is_unsigned_type(typeof(T)) ?				\
> +		(x) < 0 || (x) > type_max(typeof(T)) ? 1 : 0	\
> +		: (x) < type_min(typeof(T)) ||			\
> +		  (x) > type_max(typeof(T)) ? 1 : 0)
> +

Can't all these instances of "foo ? 1 : 0" be simplified to "foo"? That
would improve the readability of this thing somewhat IMO.

Rasmus